The present invention relates to a secondary ion mass spectrometer and more particularly to an improvement in the secondary ion mass spectrometer, which irradiates a solid sample with an accelerated positive or negative primary ion beam suitable for analyzing the composition of a metal or a semiconductor material, which draws secondary ions released from the surface of the sample into a detection system by means of an electrical potential between the sample and the detection system and which analyzes the target composition by mass separation.
Conventional secondary ion mass spectrometer irradiates a solid sample with an accelerated positive or negative primary ion beam and draw secondary ions released from the surface of the sample by irradiation into a detection system by means on an electrical potential between the sample and the secondary ion detection system (the extraction electrical potential), and analyzes the composition of the target on the basis of the secondary ions. Such spectrometers have been used to analyze the components and the composition of metallic or semiconductive material and particularly for analysis of microareas, trace analysis, analysis of depth distributions, and analysis of two-dimensional distributions.
In the case of analyzing depth distribution by means of this secondary ion mass spectrometer, the information for a depth distribution may be obtained by continually irradiating a single point or area of the target. In the case of multielement analysis multielement information for the multi-element may be obtained by successively measuring for each atomic mass in sample. Analysis of multielement depth distribution involves repetition of this operation.
The mechanism by which secondary ions are formed in this secondary ion mass spectrometer is not well understood, but generally there is a tendency for elements having high ionization potential to become negative secondary ions, and element having low ionization potential to become positive ions. Thus, we may measure for each element with high sensitivity as either a positive or a negative ion. However, the absolute intensity of secondary ion beam itself is meaningless because the intensity of secondary ion beam is highly sensitive to the surface state of the sample and the quality of the vacuum, etc. Usually, the intensity is expressed as the ratio of the intensities of secondary ions of the subject element and of the matrix element, such as Fe in the case of steel material and Si in the case of a semiconductor material, etc. (for example, I(M.sup.+)/I(Fe.sup.+), I(M.sup.+)/I(Si.sup.+), I(Cl.sup.-)/I(Fe.sup.+), etc.).
The secondary ion mass spectrometer consumes a very small amount of the sample during analysis, but analysis is destructive. For that reason, it constitutes a characteristic feature that it is necessary to rapidly control the mass the system of mass analysis secondary ion in order to obtain good results.
However, the extraction potential of secondary ion is constant by force in prior art secondary ion mass spectrometers. Therefore, in the prior art, it has not been possible to detect secondary ions of opposite polarity at the nearly same time, because the polarity of detected secondary ions was fixed to be positive or negative and the polarity of the secondary ions of matrix element such as Fe.sup.+ or Si.sup.+ or the like had to be selected so that the secondary ions can be released easily. Nevertheless, in practice it is usually important in analysis to measure the intensity of beams of the secondary ions such as Cl.sup.- or F.sup.- or the like which ionize in only the opposite polarity.
In this case, the following problems have arisen in prior art secondary ion mass spectrometers because the polarity of extraction potential of secondary ions could only be switched manually. (1) The polarity of many operating potentials, such as extraction potential of secondary ions, the electrical potential of the mass analyzing system of the secondary ions and the electrical potential of the detection system, etc., must occasionally be switched and the polarity switch takes times. Accordingly, the sample is continuously being destroyed by irradiation with primary ions while the polarity is being switched, and the measurement is interrupted during this time, or the measurement process is delayed in cases where the primary ion beam is turned off during this time. (2) The deviation of electric field which results from switching the polarity also affects the primary ion beam, and the irradiative location may shift as shown in FIGS. 10 and 11. Therefore, the primary ion beam may miss a targeted microarea, the image of a two-dimensional distribution may also be distorted, and it is impossible to carry out analysis of depth distribution of both polarities at a single point.
In FIGS. 10 and 11, 10 represents a primary ion beam generation system which produces positive (FIGS. 10 and 11) or negative primary ion beams 14 and 16 represent a solid sample and a secondary ion beam released from the surface of the sample respectively. 18 represents a mass spectro-analysis system which performs mass separation for the secondary ions released by the sample.
The conventional way to compensate for the divergence of the irradiative location of the primary beam due to switching of the polarity is to shift the sample 14 after switching, but this is time-consuming. Furthermore, it is necessary to equip the secondary ion mass spectrometer with an accurate apparatus for viewing the sample, such as an optical microscope or the like, and an accurate apparatus for slightly moving the sample. However, it has been difficult to view the sample and slightly move the sample by the conventional apparatus installed in the conventional spectrometer. It is also impossible to fully accurately correct the beam location. Furthermore, an additional problem has been that it was nearly impossible to switch the polarity repeatedly, because each polarity switch-over requires a lot of labor.
On the other hand, it is known that secondary ions of both positive and negative polarities can be measured at the same time, but in this case two mass spectroanalyzers or two secondary ion detection systems are required. Therefore, such systems have the drawbacks of being not only exceedingly expensive, but also intricate and bulky.